1. Field of the Invention
The present invention relates to telephone line terminal equipment (e.g., modems) and, in particular, to setting the appropriate DC current on the line to seize the line.
2. Description of the Related Art
Telephone lines are widely used for various forms of communications, including digital communications between computers such as PCs, with the use of terminal equipment such as modems. Modems are coupled between a computer (digital data source and sink) and a telephone line, and typically include various components such as a digital signal processor (DSP), CODEC (coder/decoder), line modulator (LM), and the like. The CODEC receives from the DSP the digital signal which is to be modulated and then transmitted by the line modulator. The CODEC therefore typically comprises a digital-to analog converter (DAC) and associated circuitry which processes the analog signal received from the DAC to provide a suitable analog voltage signal to the input of the LM. The LM, which is a voltage-controlled current source, then modulates an output current in accordance with this analog voltage signal.
Many of these components, such as the CODEC and DAC, are powered by the telephone line itself. When a modem is to be used to communicate with a remote modem via the telephone line, the LM is coupled by a suitable switch to the telephone lines, thereby providing power for these components. At this time, the telephone line is xe2x80x9coff-hook,xe2x80x9d i.e. in use.
One of the common system requirements for telecommunications systems is that, when a telephone line goes off hook, the DC current level on the line must reach a certain level within a specified period of time and maintain that level until the call is completed. The DC current level on the line must stay at a certain level in order to be interpreted by the telephone system (e.g., the telephone system central office (CO) to which the line is coupled) as an active line throughout the duration of the telephone call. (The current rise time and maximum current level are also specified in telecommunication systems to prevent damage to telecommunication equipment.) Asserting th appropriate DC line current within the specified time period is sometimes referred to as seizing the line.
When a modem is used, it must therefore seize the line within a specified time, in order to establish a telecommunications path or circuit. The time period from the time the LM is powered up (by coupling it to the telephone line so that it goes off hook) until the required DC line current is asserted by the LM may be referred to herein as the DC setup time. The specified time period during which acceptable DC line current must be asserted may thus be referred to as the maximum DC setup time. Thus, a modem must have a DC setup time that is less than the applicable maximum DC setup time in order to establish a call.
Telephone systems throughout the world have unique system requirements that need to be followed in order to legally sell and use telecommunication devices within their respective borders. These different systems have varying maximum DC setup times. One solution to this problem involves using a standard or typical modem with a variety of external components which can be switched in and out based on specific country requirements. However, this can be inconvenient and costly.
Thus, if a modem in use in a given country or telephone system is to be able to communicate with modems in other telephone systems without the necessity to change external components, the modem must have a DC setup time less than that of the smallest maximum DC setup time of all telephone systems. Currently, this smallest maximum DC setup time is 20 ms. Thus, a modem having a DC setup time less than 20 ms is compatible with the telephone systems of all countries. It is, therefore, desirable for a given modem to achieve a DC setup time in less than some specified maximum, to achieve compatibility with diverse telephone systems and to minimize the need to change equipment or configurations to place calls to any of the telephone systems.
A circuit provides a modulation signal to an input terminal of a line modulator which places a line current modulated in accordance with the modulation signal on a telephone line. An amplifier of the circuit amplifies an analog input signal to provide the modulation signal at an output terminal. A first resistor and a first capacitor are coupled in series between a first input terminal of the amplifier and the line, and a second resistor and a second capacitor are coupled in series between a second input terminal of the amplifier and the line. First and second precharge amplifiers are used to precharge the first and second capacitors, respectively, to reduce DC setup time.